The invention relates to security devices for securing articles and documents of value such as banknotes, cheques, identity documents such as passports, and documents used for brand protection etc.
A wide variety of security devices have been used in the past, some applied directly to articles or documents and others provided in the form of labels which are then transferred onto the articles. These security devices are typically in printed and/or embossed form and define many different security designs. Other security features are embedded into documents during their manufacture such as watermarks and electrotypes.
U.S. Pat. No. 6,505,779 describes the provision of security indicia formed by at least partly transparent windows formed through a security document and which are detectable in transmitted light. Indicia are provided within the bounds of a security pattern acting to visually conceal the security indicia in reflected light. This invention is based on the principle that it is possible to hide or conceal small amounts of information within larger and visually confusing information structures. A disadvantage of this technique is that it utilizes a relatively large area of the document and also does not completely hide the security indicia.
There is a continuing need to provide new security features which are more difficult for counterfeiters and fraudsters to reproduce.